Human metabolism can be monitored by means of so-called indirect calorimetry wherein, by measuring the contents and flow rate of respiratory gases, it is possible to determine oxygen consumption (V.sub.O.sbsb.2) and carbon dioxide output (V.sub.CO.sbsb.2) On the basis of these, it is further possible to calculate an estimate for daily energy consumption as well as respiratory quotient (RQ=V.sub.CO.sbsb.2 /V.sub.O.sbsb.2) which indicates the type of nutrition burning in the body. When burning carbohydrates, the amount of carbon dioxide produced is equal to the amount of oxtgen consumed, whereby RQ=1 while on fat the corresponding ratio RQ=0.7.
Indirect calorimetry is applied especially in the intensive care wards of a hospital for the quantitative and qualitative estimation of the nutritiqn demand of intravenously fed critically ill patients. Quite a few of these patients are placed in a respirator, the collection of exhalation gas to be measured being simple. However, the measuring involves several technical difficulties that have impeded spreading of this method in routine clinical application. Lately, there has been a dramatically growing scientific and also practical clinical interest to apply indirect calorimetry also to patients that are severely ill but capable of breathing on their own. This includes e.g. patients who suffer from cnacer and various metabolic diseases.
One apparatus intended for measuring patients connected to a respirator has been disclosed in Finnish Patent application No. 844562. By virtue of a constant flow aspirator included therein, said apparatus can be readily modified for applying it also to the measurement of self-respiring patients, as long as the collection of respiratory gases can be performed without substantially disturbing the patient.
The collection of respiratory gases is generally effected by means of a tight mask covering the nose and mouth or by means of a tube placed in the mouth, in which case the nose is closed by a separate clamp. However, the use of these instruments has been found (1) to disturb a patient to such a degree that respiration changes decisively and a state of balance required for the reliable measurement of metabolic variables cannot be reached during the time a patient can usually tolerate said instruments on his or her face. Various solutions have been developed for this problem, wherein e.g. the head of a patient is placed in a closed or half-open box with a continuous airstream passed therethrough. A particularly known device is "a canopy" (2), developed by Prof. John M. Kinney at the Columbia University, New York, which is an acrylic-made, transparent, hinged, rectangular box associated with a separate element to be sealed around the neck of a patient. Typical of this solution is that the box has such a great volume and is of such a design that the exhalation gas is evenly distributed in the space prior to leaving it.
Thus, the patient is forced to re-inhale rather high carbon dioxide contents. If the air flow passed through the box is 50 l/min and the patient has V.sub.CO.sbsb.2 =250 ml/min, the CO.sub.2 -content will be 0.5%. Since the measurement of gas contents this low at a high accuracy is difficult, an attempt is made to keep the air flow as low as possible for reaching higher CO.sub.2 -contents to be measured.
According to Prof. Kinney, the accepted maximum content of CO.sub.2 is 1%. Above this, the carbon dioxide begins to stimulate respiration and, in view of metabolic measurements, the results begin to vary. On the other hand, according the subjective experience encountered by patients, considerably lower CO.sub.2 -contents can bring about a constricting sensation and change respiration.